Syria’s leadership has said the country, which is believed to have nerve agents as well as mustard gas and Scud missiles capable of delivering them, could use chemical or biological weapons if it were attacked from outside.

President Barack Obama has called it a “red line” for the U.S. if Assad’s regime were to use such lethal weapons, and France has been ratcheting up its language on the issue.

If Syria uses such weapons, “our response … would be massive and blistering,” French Foreign Minister Laurent Fabius said on RMC radio Monday.

He said Western countries are monitoring the movement of the weapons in Syria to be ready to “step in” immediately.

Fabius said “we are discussing this notably with our American and English partners.”

Fabius added that Russia and China are “of the same position,” but did not elaborate. Fabius acknowledged frustration at their continuing support for Assad.

The foreign ministries of both China and Russia declined immediate comment on Monday.

Since the start of the Syrian conflict, Beijing has been consistent in its stance that it should be settled through negotiations and not by outside forces.

Moscow is Syria’s chief ally, and Russian Deputy Foreign Minister Gennady Gatilov told the AP in a recent interview that Russia has the Syrian government’s assurances that chemical weapons will not be used. Gatilov said Russia will “work toward the goal of preventing such things from happening.”

China and Russia have repeatedly used their veto powers in the U.N. Security Council to block U.S.- and Arab-backed action that could have led to sanctions against Assad’s regime.

Syria’s opposition has urged outside military help against Assad’s armed forces.

“I am going to be very clear, we are requesting military intervention in order to protect Syrian civilians who have been constantly murdered over the last year and a half,” the head of the Syrian National Council, Abdelbaset Sieda, said in Madrid on Monday.

“The European Union should take the initiative and pressure Russia … so we can lay down some protected areas for refugees,” Sieda said after meeting Spain’s Foreign Affairs Minister Jose Manuel Garcia Margallo.

Margallo said Spain would continue to press for a united EU approach to Syria and urged Syrian opposition groups to overcome their divisions and join together against the Assad regime.

The new U.N. envoy to Syria, Lakhdar Brahimi, acknowledged Monday that brokering an end to the civil war is a “very, very difficult task.”